The present invention relates to a polypropylene-based multilayer film which exhibits improved moisture barrier properties and enhanced mechanical properties.
Polymeric films are used in many commercial applications. One particularly important application is the packaging of food products. Films employed in the food packaging industry are chosen and/or designed to provide characteristics necessary for proper food containment. Such characteristics include water vapor barrier properties, oxygen and gas barrier properties and flavor and aroma barrier properties.
Polypropylene is a polymer commonly employed in the manufacture of films used in the food packaging industry. In the case of multilayer films, polypropylene is typically used in the base or core layer. Often the polypropylene layer is modified to obtain desired characteristics which are not inherent in unmodified polypropylene. For example, resin modifiers and/or additives may be blended with the polypropylene.
Attempts to vary the characteristics of polypropylene are often directed towards improving the moisture barrier of the resultant film. To this end, it is known that blending polypropylene with a resin modifier will provide such improvements. Typically, from about 10% to about 20% resin modifier must be added to the film to achieve the desired reduction in water vapor transmission.
However, the addition of the resin modifier, within the aforementioned ranges, is not without its disadvantage. Particularly, at the loading ranges typically employed in the art, the polypropylene suffers a significant decrease in dimensional stability. This in turn hinders the machinability and processability of the resultant films, resulting in increased manufacturing costs and/or films of inferior quality.
The mechanical properties of a polymeric film are another important characteristic, particularly with respect to such applications as wrappings for tobacco products. Films having enhanced mechanical properties facilitate handling and packaging because such films are more readily accommodated by typical industrial machinery. Attempts have been made to enhance the mechanical properties of polypropylene-based films (as measured by the stiffness and moduli of the film) through increased orientation and/or by the addition of additives. However, increased orientation often increases the likelihood of film splitting during manufacturing, while the addition of additives typically provides limited enhancement of mechanical properties but can negatively impact other film characteristics such as dimensional stability and clarity.
Commonly-owned copending U.S. patent application Ser. Nos. 08/490,081 filed Jun. 13, 1995 and 08/542,209 filed Oct. 12, 1995 disclose polymeric film structures having a base layer of high crystallinity polypropylene and up to about 8% by weight of a resin modifier. Skin layers may be adhered to opposing surfaces of the base layer. The disclosed film structures exhibit improved moisture barrier properties and enhanced mechanical properties, while maintaining dimensional stability, machinability, processability and clarity. However, the use of high crystallinity polypropylene in the disclosed film structures increases the difficulty of manufacturing the structures, particularly with respect to such manufacturing steps as stretching and slitting of the films. The use of high cystallinity polypropylene also increases the cost of the film structures.
Thus, there is still a need in the art for a resin modified polypropylene-based film which may be readily manufactured at reduced cost, while exhibiting improved moisture barrier properties, enhanced mechanical properties, dimensional stability, machinability, processability and clarity.